This invention relates to a micromechanical memory sensor. More particularly, the invention is directed to a micromechanical device which serves as a mechanical memory latch or sensor, the activation of which is triggered by a change of conditions, e.g., temperature, acceleration and/or pressure. Contents of the memory latch can be conveniently detected at any time after latching. The device is electronically resettable so that it can be conveniently reused.
While the invention is particularly directed to the art of micromechanical memory sensors, and will be thus described with specific reference thereto, it will be appreciated that the invention may have usefulness in other fields and applications.
Micromechanical memory sensors are used or have potential use in sensing a variety of different variables or conditions. These variables or conditions include temperature, acceleration, pressure, force . . . etc.
For example, a micromechanical memory sensor adaptable for use in sensing temperature extremes purely mechanically and being electronically resettable would be advantageous for applications wherein field testing is conducted on products and no power supplies are available in the field. However, there are no known micromechanical temperature sensors of this type.
Conventional electronic temperature sensors require a power supply when monitoring temperatures. However, in most instances where the temperature extreme to which a product has been exposed is the desired information, the field monitoring of temperature is not possible with conventional techniques since a power supply may oftentimes be unavailable.
A bistable snapping microactuator having a power supply, or battery, has also been disclosed. H. Matobo, T. Ishikawa, C. Kim, R. Muller, A Bistable Snapping Microactivator, I.E.E.E., January 1994, pp. 45-50. The microactuator includes a flexible cantilever which buckles when a temperature extreme, induced by a current, is detected. While this device is ultimately triggered by a temperature change, i.e., resistive dissipation, acceptable operation is only achieved through the use of driving voltages and current pulses applied in a particular timing sequence. This microactuator is not triggered purely mechanically.
As a further example, certain micromechanical memory sensors adapted for use as latch accelerometers are known and provide an inexpensive way of sensing moderate and high-g accelerations by using a micromechanical memory sensor. A latch accelerometer is a switch which latches if accelerated by a predetermined acceleration in a particular direction and remains closed after the acceleration ceases. The primary advantage of latch accelerometers over the conventional acceleration sensing devices is that latch accelerometers do not require complicated sensing electronics. The sensed acceleration can be read out long after the accelerating event. Acceleration latches operate without a power supply and can be made to operate at g levels ranging from only a few g's to several thousand g's and to sense the duration for which the acceleration lasts.
U.S. Pat. No. 4,891,255 to Ciarlo discloses an acceleration latch which uses bulk micromachining of (110) oriented silicon wafers to make two cantilever beams having proof masses, or plates, attached thereto that interlock at a set threshold acceleration. FIGS. 21(a) and 21(b) herein respectively show such a latching accemlerometer similar to that shown in FIGS. 3-4 of the Ciarlo patent. The cantilever beams C are typically several millimeters in length. The fabrication of the cantilever beams C and the proof masses P is fairly complicated since corner compensation and silicon bulk micromachining of (110) wafers are used. (110) bulk micromachining is not readily compatible with IC processing.
The cantilever beams C of the Ciarlo patent must undergo large deflections before latching at their proof masses C. Further, since the horizontal cantilever beam C must force deflection of the vertical cantilever C, which involves the sliding of the two large surfaces, the frictional force between the two proof masses P can be significant and can result in uncertainties in the acceleration sensed. Moreover, the cantilever beams C are not delatchable, thus not resettable.
Another main disadvantage of the latch of the Ciarlo patent is the complicated readout schemes that must be used. Since the cantilever beams C are made by etching through a silicon wafer, the two cantilever beams C cannot be electrically isolated, making a simple continuity test between the two cantilever beams C impossible. The readout schemes of the Ciarlo patent use either capacitive or optical techniques. In either of these schemes the accelerometer wafer must be sandwiched between two other wafers containing capacitive plates or light emitting diodes to sense the position of the cantilevers. This makes the fabrication process much more complicated and expensive. Also, bulk micromachining results in large sized devices.
A direct implementation of the latching mechanism of the Ciarlo patent using surface micromachining is possible and may solve the problem of sensing the latch. However, the device would still suffer from other noted problems related to excessive length of beams C with the proof masses P attached at ends thereto and would still not be resettable.